1. Field of the Invention
The present invention relates to a mobile communication terminal such as a mobile telephone set provided with a transmission unit which can switch a frequency band, and more particularly to an apparatus for switching a matching circuit to connect and disconnect an impedance matching element to an RF signal input and output line in accordance with a frequency band switching-signal.
2. Prior Art
FIG. 1 shows a conventional apparatus for switching a matching circuit in a mobile communication terminal.
In FIG. 1, an RF input terminal 1 is connected with an RF output terminal 2 via a first coupling capacitor 5 and a second coupling capacitor 7 to form an RF signal input and output line. A PIN diode B for switching a frequency band is connected in parallel between the first and second coupling capacitors 5 and 7 at its cathode to the RF signal input and output line, and a grounded limitation resistance 3 is also connected in parallel thereto.
The anode of the PIN diode 8 is connected with a matching capacitor 9 as an impedance matching element and a choke coil 10 is connected in parallel to a midpoint of them, while an RF bypass capacitor 11 is connected in parallel to the choke coil 10 and a frequency band switching-signal input terminal 12 is connected in series thereto.
When the frequency band switching-signal which is input to the frequency band switching-signal input terminal 12 is H level, a voltage of the frequency band switching-signal is applied via the choke coil 10 as a DC voltage to the PIN diode 8, and the PIN diode 8 is turned on to make an impedance between the anode and the cathode low at the RF signal.
Consequently, the RF signal input and output line communicating from the RF input terminal 1 to the RF output terminal 2 is added with the matching capacitor 9, and the impedance is optimized at a selected band.
In addition, the choke coil 10 is grounded on the side of the frequency band switching-signal input terminal 12 via the RF bypass capacitor 11 so that an RF impedance becomes high on the side of the choke coil 10 at the anode of the PIN diode 8 to result in no influence on an impedance of the RF signal input and output line communicating from the RF input terminal 1 to the RF output terminal 2.
Meanwhile, when the frequency band switching-signal is L level, the PIN diode 8 is turned off and an RF impedance becomes high at the cathode of the PIN diode 8, so that the matching capacitor 9 is insulated from the RF signal input and output line communicating from the RF input terminal 1 to the RF output terminal 2, because no voltage is applied to the anode of the PIN diode 8.
In the conventional apparatus, however there is a disadvantage in that there is a time for which the PIN diode 8 is turned on during a period of an off-time by a negative amplitude of a transmission power, so that a precise switching of a frequency band is deteriorated, when a large amplitude of a transmission power is applied to the cathode of the PIN diode 8, because the RF amplitude waveform on the cathode terminal of the PIN diode 8 at an off-time is of an amplitude changing on the central axis of 0V as shown in FIG. 3.
In order to carry out a precise switching operation of a frequency band in the past, a GaAs switching-element which is very expensive must be used as a frequency band switching-element in place of a PIN diode.